Include igraph library

[borassi]

Include igraph library ![1] in Sagemath, so that we can use its algorithms. Add routines to convert Sage graphs into igraph and viceversa.

If we manage to include igraph, we will have access to 62 algorithms on graphs: 29 of them are not available in Sage, yet ![2].

![1]​http://igraph.org/python/

![2] ​https://docs.google.com/spreadsheets/d/1Iu1hkQtRn9J-sgfZbQTu2RoXzyjoMEWP5-cm3nAwnWE/edit#gid=0

[borassi]

Hello!

With this ticket, I would like to add igraph library to Sage. I have tried to follow as much as possible the instructions in ![1], to include the following external libraries:

• igraph-0.7.1, available in ![2] (the C/C++ core of igraph);
• python-igraph-0.7.0, available in ![3] (the Python interface).

In order to build this code, you should do the following.

• Download the two .tar.gz source code files [2,3] and put them in folder SAGE_ROOT/upstream (this folder is not synchronized with git).
• run the script SAGE_ROOT/build/pkgs/python_igraph/spkg-src from folder SAGE_ROOT, with command ./build/pkgs/python_igraph/spkg-src. The problem is that Sage does not like dashes inside the package name: the script converts the file python-igraph-0.7.0.tar.gz into python_igraph-0.7.0.gz (it is also necessary to modify the name of a folder inside the tar archive).

I am not really sure about the procedure (in particular, about spkg-src), so I would like to have feedback, also related to ticket #18826.

Thank you very much!

Michele

![1] doc.sagemath.org/html/en/developer/packaging.html

![2] ​http://igraph.org/c/#downloads![3] ​http://igraph.org/python/#downloads